Pedigrees were constructed from the family histories of all patients participating in the dementia program in order to examine the genetic basis of Alzheimer's disease. Differences in family history between monozygotic twins discordant or concordant for dementia of the Alzheimer type suggest heritable and nonheritable forms of Alzheimer's disease. Collaborative studies were established to examine the ability of peripheral blood lymphocytes of probands with Down syndrome and familial Alzheimer's disease to repair X-irradiation induced damage during the G2 period of the cell cycle, and for the Down syndrome subjects, to see if the parents' lymphocytes show chromosomol instability. Efforts continue to evaluate genetic aspects of presenile dementia. Specifically, secondary sex chromosomal variation, alpha-l-antitrypsin (PI) phenotyping, and cytological analysis of variations in the Nucleolus Organizing Regions (NOR) were analyzed. Hind III digests of southern blots of genomic DNA from 4 sets of identical twins were obtained. Analysis with the polymorphic DNA marker D2lS52 showed that all sets of twins were identical.